1. Field of the Invention
The present invention relates to an optical writing device, an image forming apparatus and a light quantity correcting method.
2. Description of Related Art
In recent years, as an optical writing device to form an electrostatic latent image on the surface of a photosensitive body, an image forming apparatus using a light emitting diode (LED) printer head (hereinafter referred to as LPH) has been developed. The LPH is composed of LED chips arranged in an array, an optical section, such as a graded-index (GRIN) lens, and the like. Each of the LED chips includes a plurality of LED elements arranged according to previously set resolution along a main scanning direction. The optical section condenses irradiated light emitted from the LED elements according to image data to form an electrostatic latent image on the photosensitive body.
It is known that the LPH described above produces light quantity unevenness in association with the manufacturing dispersion of the LED elements, the optical characteristics of the GRIN lens, and the like. In order to settle the light quantity unevenness, a technique of performing optical writing is known. The technique digitally controls the current values of the driver circuits which illuminate the LED elements to previously store the light quantity correcting data so as to make the light quantities of the plurality of LED elements to be uniform, into an electrically erasable and writable nonvolatile memory, such as an electronically erasable and programmable read only memory (EPROM). Then, the technique reads the light quantity correcting data stored in the memory to a control apparatus, which collectively controls the image forming apparatus, and performs the optical writing by using the image data and the read light quantity correcting data.
Moreover, the heightening of the resolution in the main scanning direction (the arrangement direction of the LED chips), such as 600 dpi or 1200 dpi, has been advancing in association with the realization of the densification of the arrangement of the LED elements. For example, when the maximum paper size capable of image formation in an image forming apparatus is 324 mm (width direction) of the A3 wide size, then 7680 LED elements are arranged in the case of the resolution of 600 dpi, and 15360 LED elements are arranged in the case of the resolution of 1200 dpi.
In association with the increase of the number of LED elements accompanying the heightening of the resolution in this manner, the quantity of the data to be controlled as image data increases, and the optical writing control method also comes to control an exposure time of lighting on the basis of the set numerical values of a plurality of bits as well as the simple on-off operations of the LED elements. The traffic of data to be used for performing optical writing has also increased. Consequently, it has become indispensable to install an LPH interface having a large capacity and equipped with a high-speed data transmission function in association with the demand of the improvement of the productivity (high-speed performance) of the image forming apparatus.
Furthermore, when images are formed on various kinds of paper, for example, an electrophotographic printing system image forming apparatus must meet data communication functions dealing with the image formation speeds according to various types of paper by being equipped with the plurality of image formation speeds according to various paper features (such as paper types and thicknesses) even in the same image forming apparatus in order to improve the fixation property of toner.
To settle the problem mentioned above, there is a technique to attain a high-speed conversion of multi-bit data as a technique of an interface having a large capacity and realizing a high-speed data transmission function. The technique, for example, performs the parallel-serial conversion of clock synchronous parallel data with a low voltage differential signaling (LVDS) circuit and performs the clock modulation of the converted data according to the number of serial conversion bits with a phase locked loop (PLL) circuit on a transmission side, and the technique restores the modulated serial data to the input parallel data by the serial-parallel conversion thereof with a receiver circuit equipped with a frequency modulation circuit and by restoring the modulated clocks to the document ones on a reception side. The technique thereby attains the high-speed conversion of the multi-bit data.
The large capacity and high-speed data transmission function can be realized by arranging a control signal, image data, and light quantity correcting data in parallel data by means of the aforesaid technique, and the degree of freedom of a bundled wire length is enhanced to enable the degree of freedom of the layout of the inside of an image forming apparatus. Thus high-speed data processing sections can be arranged in a concentrated manner to be a unit.
However, the method of settlement mentioned above can perform high-speed communications of exposure data and the like to an LPH, but, when the method is tried to utilize for a reading section of the light quantity correcting data of the LPH, then the cost of the circuit parts thereof rises, and the production cost rises in association with the rise of the cost of the circuit parts. Consequently, the rise of the production cost causes a disadvantage for a user. Moreover, there is a limitation of a bundled wire length owing to the limitation of the circuit configuration to read the light quantity correcting data, and the method has the problem of the impossibility of making the best use of the performance of the LVDS circuit for exposure data.
There is also the method of installing a memory (such as a ROM) storing the light quantity correcting data in an image forming apparatus as a means of settling this problem, but, because the image forming apparatus, which is required to be a high speed and to have high durability, needs to exchange the LPH and adjust the light quantity correcting data according to the process conditions and the frequency of use thereof at the time of the maintenance thereof, it is necessary to perform the updating of the data of the memory or the exchange of the memory for every operation of the exchange and the adjustment. Consequently, when the memory does not store the light quantity correcting data fitted to the LPH owing to a minor operation mistake, then the fact becomes a cause of producing a bad image. Furthermore, the management of the light quantity correcting data becomes necessary also in the production process of the image forming apparatus, and it becomes necessary to collate the LPH installed in the image forming apparatus with the light quantity correcting data stored in the memory to cause a new technical problem.
Accordingly, a technique of providing a nonvolatile memory storing light quantity correcting data in the LPH to read the light quantity correcting data from the memory is generally performed.
For example, Japanese Patent Application Laid-Open Publication No. 2001-239697 discloses an apparatus which performs the light control of LED elements by reading light quantity correcting data from an EEPROM (a memory storing the light quantity correcting data) through a strobe signal, by supplying the read light quantity correcting data to an LED driver IC as print data, and by supplying a drive instruction of a LED array according to the print data by the strobe signal as the selection signal of an LED array group.
The technique disclosed in Japanese Patent Application Laid-Open Publication No. 2001-239697 describes that a drive section (printing control section) generates a clock signal to obtain the light quantity correcting data, and inputs the light quantity correcting data stored in the memory into the drive section in synchronization with the generated clock signal, and further transfers the light quantity correcting data by the supplied clock signal. The technique aims at obtaining the effect of reducing the design margin accompanying a timing change between the transfer clock of the printing data and the transfer clock of the light quantity correcting data. That is, the technique individually considers the reading control of the light quantity correcting data, the setting control of the light quantity correcting data, and the transmission method of printing data, and provides an interface circuit balancing so that each of them can be processed by a proper method.
Moreover, the similar technique to a detachably attachable part unit as well as the LPH exists, and, for example, Japanese Patent Application Laid-Open Publication No. 2004-053761 discloses an image forming apparatus to read the information stored in a memory device of a part unit, which is exchangeably installed in an image forming apparatus, before the installation of the memory into the image forming apparatus with a reading section provided in the image forming apparatus.
Furthermore, Japanese Patent Application Laid-Open Publication No. 2005-141044 discloses a printing apparatus to read exchange notice information from an RFID tag in noncontact with it with a single antenna. The RFID tag is disposed in the image formation unit of each color to be printed, and stores the exchange notice information to notify an exchange instruction of each image formation unit beforehand.
However, because the related art mentioned above is required to balance the interfaces of the whole image forming apparatus in order to improve the throughput of large capacity light quantity correcting data and the communication performances (high-speed performance and reliability of transmission signal) of data, the printing performance thereof is sacrificed. Consequently, the communication performance is limited by the reading of light quantity correcting data from the memory and a set function. Moreover, in the case of the technique of Japanese Patent Application Laid-Open Publication No. 2001-239697, also in the case where the bundled wire length is desired to be elongated, the bundled wire length is limited by the interface circuit.
Furthermore, because the related art mentioned above provides a memory to each part, that is, to each LPH, it is difficult to read or write light quantity correcting data against an arbitrary LED element, and must perform the reading or writing of light quantity correcting data to all of the LED elements provided in the LPH. Consequently, when image formation is performed to a sheet of paper having a smaller width than an effective light writing width, then it is sufficient to read the light quantity correcting data of the LED elements to the width of the paper. But, the light quantity correcting data of all the LED elements is read. Consequently, the related art has the problem of the generation of a useless communication load.